This invention relates to novel microorganisms having a conversion ability to bile acids and a position-specific oxidation method of the hydrocyl group of the di(or tri)-hydroxycholanic acid. More specifically, this invention relates to microorganisms having a conversion ability to bile acids and belonging to the genus Bacillus, and a method for preparing, using such a microorganism, 3.alpha.-hydroxy-7-keto-5-.beta.-cholanic acid, or 3.alpha., 12.alpha.-dihydroxy-7-keto-5-.beta.-cholanic acid or 3.alpha.-hydroxy-7,12-diketo-5.beta.-cholanic acid, each of which is an useful intermediate for preparation of 3.alpha.,7.beta.-dihydroxy -5-.beta.-cholanic aicd (hereinafter referred to as ursodeoxycholic acid) useful as a cholagogue, from 3.alpha., 7.alpha.-dihydroxy-5-.beta.-cholanic acid (hereinafter referred to as chenodeoxycholic acid) or 3.alpha.,7.alpha.,12.alpha.-trihydroxy-5.beta.-cholanic acid (hereinafter referred to as cholic acid).